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Glossary
Aliquot
Analyte
Bell curve
Blank
Bulk sample (or
sample)
Determinate error
Gaussian distribution
GLP
Half-life
A portion of the "bulk sample".
The particular element or compound which is to be quantified.
See Gaussian Distribution.
A sample which has been prepared that contains no analyte.
The material provided for analysis.
A ‘systematic’ error. Systematic errors can have a variety of origins: for example, a
single measurement might be wrong because the instrument baseline was not
zeroed between samples, or the balance used for weighing materials might have
caused all weights to be too high. Systematic errors can have other causes; a stock
solution of the wrong concentration might have been used, or there may be a
limitation in the background theory, such as the assumption that the ideal gas law,
pV=nRT, is exact. Errors can also arise through repeated error by the chemist. For
example, most pipettes are not of the "blow-out" type; the liquid in these pipettes
must be allowed to drain, but a small, and reproducible, amount of liquid remains
in the tip. If you blow out this liquid each time you will introduce an error to the
concentration of every solution.
Often also called a normal distribution, or a Bell curve. A Gaussian distribution will
arise if the experimental errors are truly random. When that is the case, the
readings will cluster around the mean value and the curve will be symmetrical.
Deviation from a symmetrical shape indicates the presence of systematic error.
Good Lab Practice is a set of controls and specifications that define how an
experimental procedure is to be carried out. If followed, the procedure ensures
that the results of the experiment can be relied upon, within specified error limits.
Normal analytical experiments should be carried out according to the principles of
GLP.
The time required for the concentration of a species to fall by 1/2 ; similar to, but
not identical to the lifetime.
Indeterminate error
Knurl
Lifetime
Method spike
Method blank
Multistandard
QC
QC sample
Sample
Sample spike
Spike
An indeterminate error is a ‘random’ error. These appear due to random variations
in preparation and measurement of samples. In most types of experiment the
distribution of such errors is generally well described by a "normal" or "Gaussian"
distribution. The calculation and use of certain statistical measures, such as
standard deviations, depends upon random errors forming a Gaussian distribution.
To provide with ridges, to assist the grasp, as in the edge of a flat knob, or coin; to
mill.
The lifetime of a species whose concentration is changing is the time required for
the concentration to fall to 1/e of its initial value. Lifetimes in chemistry vary over
a large range, from around 10-16 s for some very short-lived atomic states to
billions of years for elements that are weakly radioactive. See also half-life.
A spike sample made up using the same sample preparation procedure that you
used for your ‘unknown’ sample (i.e., all the steps required by the method are
carried out when preparing the spike). The method spike contains all the
required reagents, plus a known quantity of analyte.
A method blank is made up using the same sample preparation procedure used for
your ‘unknown’ sample i.e., all the steps required by the method, but it only
contains the necessary reagents.
Most analytical standards contain just a single analyte. However, in some
analytical procedures more than one analyte is measured, and it is then common
to use multistandards. A multistandard contains several analytes in a single
solution, so a set of multistandards allows one to prepare several calibration
curves. Multistandards may be prepared by making a single stock solution
containing all analytes, then diluting this to make a series of more dilute solutions,
in each of which the ratio of different analytes will be the same as the ratio in the
stock. Alternatively, each multistandard may be prepared individually, so that the
ratio between the concentrations of different analytes in the stock can vary from
one solution to the next.
Quality Control. Quality control refers to any actions taken to ensure that
analytical results are accurate and precise. Often summarized by the GLP.
The Quality Control sample is a sample of defined composition, whose purpose is
to allow you to confirm that the instrument, standards, method, and analyst are all
working correctly.
The sample contains an aliquot of the bulk sample, plus all reagents etc. It is
standard practice to prepare samples in duplicate or triplicate. The term "sample"
also refers to any portion of the bulk sample, blank or spike that is being prepared
for analysis
The sample spike is an aliquot of an ‘unknown’ sample, to which has been added a
known amount of analyte; this sample is then carried through the same sample
preparation procedure as the ‘unknown’ sample (i.e., all the steps required by the
method). It therefore contains a known quantity of sample (which itself is of
unknown concentration…) and an additional known quantity of analyte, plus all
reagents etc. The amount of material added should be sufficiently small that the
change in the matrix of the sample is minimized.
Spiking is the enrichment of a sample or blank with a known quantity of the
analyte under investigation. The amount added is usually chosen to be from
50-100% of the expected sample analyte concentration in order to ensure that,
Spike recovery
when measured, the spiked solution will lie comfortably within the range defined
by the standards.
The spike recovery is the percentage of the added analyte that is ‘recovered’, i.e.,
measured during the analytical procedure. Since the measurement will
unavoidably contain some error, there is uncertainty in the percentage, and this
may lead to a "recovery" in excess of 100%.
Split/splitless
injection
Capillary columns are the most widely used type of GC column. Chromatographic
material coats the inner surface of the column; since the column is very narrow
and the coating extremely thin, the amount of material is tiny; consequently, only
a very small amount of sample can be injected without overloading the column. In
"split" injection the sample is injected into, and evaporates within, a small heated
volume at the head of the column. One exit from this volume leads straight into
the column, while a larger exit leads to waste. The sample is thereby split into two
portions, and only a small fraction is fed into the column. If the sample is
extremely dilute, detection limits may be an issue, so the entire sample may be fed
into the column using "splitless" injection. [Figure from sge.com.]
Standard
A standard is a sample of accurately-known composition, generally containing a
single analyte in the same matrix as the unknown samples. Although the solvent
used for the standard is typically the same as that in which the unknown analyte is
dissolved, the method by which a standard is prepared is often quite different to
that used to prepare the unknowns; consequently one often needs separate
blanks to correct the signal from the standards and from unknown samples.
See also multistandards.
The standard blank is prepared using the same reagents as those required for the
standards, but need not (though it may be) prepared in the same way as the
method blanks.
Standard blank